Environmental effects, such as those due to temperature, humidity, barometric pressure, etc. may have a significant effect on the performance of force sensors such as load cell assembly. By a "load cell assembly" what is meant is a load cell body which receives an applied force, and which includes associated force sensors and driving electronics therefor for sensing, calculating, and outputting a force value representative of the applied force on the load cell body.
Many attempts have been made to reduce or eliminate the effects of environmental conditions on the output of a load cell assembly. For example, a load cell assembly may be isolated or sealed against particular effects such that the assembly is used in more carefully controlled environmental conditions. However, it has been found that completely sealing a load assembly from environmental effects is difficult, if not impossible, particularly when an assembly is used in more rigorous environments (e.g., environments with wide temperature variances).
Another manner of accommodating for environmental conditions is to eliminate or reduce the effects thereof electronically through signal processing. For example, one manner in which environmental effects may be reduced is through differential sensing, whereby a pair of force sensors are mounted in a load cell body in such a manner that they react oppositely, or complementary, to an applied force. Typically, the applied force may be due to weight, acceleration, impact, pressure, etc.
Different force sensors may be included on a load cell assembly to react complimentary to one another. For example, the force sensors may be placed in tension and compression modes, whereby an applied force will place one sensor in tension, and the other in compression. With strain gauges or other analog force sensors, the resistances of the complementary sensors will respectively increase and decrease due to applied force. Similarly, with resonator sensors such as vibrating strings and tuning forks, the resonant frequencies of the sensors will respectively increase and decrease due to applied force.
In theory, by subtracting the output of one force sensor from the output of the other force sensor (i.e., processing the differential effects), a generally reliable output signal indicative of the applied force may be obtained. Errors due to environmental effects in theory are rejected from the output signal because they tend to affect each sensor similarly (i.e., they are common mode effects), and are thus cancelled out in the subtraction operation. However, it has been found that it is difficult to manufacture complimentary sensors which have precisely matched sensitivities to common mode effects. Therefore, subtracting their output signals will not entirely cancel out the common mode effects.
U.S. Pat. No. 4,815,547 to Dillon et al. represents one attempt to improve the correction of common mode effects from a differential output of complimentary sensor signals. In the Dillon et al. assembly, a pair of strain gauges are placed in tension, and another pair are placed in compression. The four strain gauges are coupled in a bridge circuit which produces an analog differential signal that represents a percentage of a reference signal. The analog differential signal is then converted to digital form and linearized and corrected for common mode effects by applying a quadratic equation with first and second order coefficients to the signal. Temperature correction is performed using a separate temperature sensor located on the assembly.
One advantage of Dillon et al. is that the coefficients of the quadratic equation may be determined from environmental testing, then stored in a memory on the assembly to characterize the assembly without the need for any hardware modifications. However, Dillon et al. suffers from a drawback in that the system is primarily analog, which limits the possible resolution of the system irrespective of any digital signal processing that is later performed. Dillon et al. utilizes analog strain gauges, an analog bridge circuit, and an A/D convertor, all of which have limited resolution. Consequently, assemblies of this type are typically unable to exceed resolutions of 6,000 divisions over a reasonable temperature range.
On the other hand, U.S. patent application Ser. No. 08/064,551, filed on May 19, 1993 by Bell et al., now U.S. Pat. No. 5,442,146, issued Aug. 15, 1995, discloses a fully digital load cell assembly which is capable of providing much higher resolution than prior analog-based systems. The Bell et al. load cell assembly utilizes double ended tuning forks placed in tension and compression modes which may be quickly and reliably converted from frequency to digital form. Bell et al. utilizes independent digital signal acquisition and conditioning such that the sensor signals are independently retrieved, amplified and filtered. The signals are then combined in a linearization and common mode rejection routine to digitally adjust the sensitivities of the sensor outputs to remove common mode effects during the differencing operation. The individual sensitivities of the sensor outputs are modeled by a quadratic equation having calibration constants which are determined from applying known forces to an assembly and curve-fitting the data points obtained therefrom.
The Bell et al. load cell assembly uses a separate temperature correction routine that corrects for zero shift and span error due to temperature based upon calibration data determined from environmental testing and programmed into the assembly. This eliminates the need for a separate temperature sensor, as the temperature also may be extracted from the sensor output signals.
As a result, Bell et al. may provide much greater resolution than typical analog-based systems, typically about 10,000 or more divisions. The improved resolution of resonator sensors, coupled with the independent signal processing and reliable digital linearization, common mode rejection and temperature compensation, are significantly improved over prior analog-based systems.
Each of the above systems, however, requires separate corrections to be performed for various common mode effects such as temperature, humidity, barometric pressure, long term drift, etc. For example, to correct the Bell et al. load cell assembly for the effects of humidity, a separate humidity correction routine, similar to that performed for temperature correction, would be required. However, requiring separate corrections for different common mode effects would require separate signal processing for each effect, which would significantly add to the number of calculations needed for each force value update.
Furthermore, correcting for separate effects would require separate calibration procedures to determine the particular coefficients for the correction routines. For example, it is known that temperature is typically the most prevalent common mode effect. Moreover, zero shift due to temperature constitutes the most prevalent part of this effect. Calibration for temperature typically requires sample data to be taken at at least three (and preferably more) temperatures. To obtain data readings at various temperatures, each load cell assembly must be allowed to obtain a uniform temperature in a controlled environment, which may take over an hour or more. Consequently, performing calibration over several temperatures often takes at least a day for some assemblies, which typically adds to the manufacturing time and costs of the assemblies. To calibrate for other effects, such as humidity, barometric pressure, etc., similar calibration routines to that for temperature would be required, further adding to the manufacturing cost and time for the assemblies.
Therefore, a need exists in the art for a load cell assembly and routine therefor for reducing or eliminating many common mode effects in a fast and efficient manner to obtain higher resolutions. In addition, a need exists for a load cell assembly and routine therefore which may reduce or eliminate the amount of costly and time-consuming calibration procedures which are currently required.